1. Field of the Disclosure
Embodiments disclosed herein relate generally to removal of benzene from a reformate stream via catalytic transalkylation with polyalkylate in presence of a heterogeneous slurry catalyst, which can be continuously replaced during the operation.
2. Background
The demand for cleaner and safer transportation fuels is becoming greater every year. Two major sources of gasoline feedstock, including reformate and cracked petroleum feedstocks, present both a problem meeting strict environmental regulations and impose certain health risks. For example, light reformate typically contains unacceptably high levels of benzene, a known carcinogen. Heavy reformate may contains unacceptably high levels of C10 and heavier polyalkylate aromatics (polyalkylate) that diminish the value and the environmental quality of the fuel.
Refiners in the U.S. and in other countries are required to remove a substantial portion of the benzene from the reformate stream. Practical options to date include extraction, hydrogenation, alkylation, and transalkylation of benzene. Each of these options presents challenges, especially to a small or non-integrated refiner, from both a standpoint of cost and feasibility.
Extraction of benzene requires expensive capital investment in necessary equipment and a customer for the benzene product, neither of which may be feasible for a small non-integrated refiner. Also, while it is possible to extract benzene from the gasoline pool by fractionation techniques, such techniques are not preferred, because the boiling point of benzene is too close to that of some of the more desirable organic components, including C6 paraffins and isoparaffins. Monoalkylate aromatics (monoalkylate), such as toluene and xylenes, are more desirable for gasoline blending, as opposed to benzene, because they are less objectionable both from an environmental and a safety point of view.
Alternatively, benzene in reformate may be removed via hydrogenation. However, hydrogenation of aromatics, such as benzene, toluene, and xylenes, results in reduced octane rating of the reformate stream, and thus diminishes the overall value of the fuel. As with extraction, hydrogenation of benzene also may not feasible for a small refiner due to potentially uneconomical costs associated with supplying hydrogen.
Alkylation of benzene with an olefin to form a monoalkylate product is another option available to refiners. Alkylation is not as effective in upgrading the overall fuel value of reformate, because it does not affect the polyalkylate content. Additionally, alkylation requires a readily available olefin source, and therefore may not be feasible for small refiners.
Therefore, there is still a significant need in the art for methods to reduce the levels of benzene and C10 and heavier polyalkylate in refinery streams, including reformate, especially for smaller refining operations.
As taught in U.S. Pat. No. 5,053,573 and U.S. Pat. No. 5,406,016, the levels of both benzene and polyalkylate contained in refinery streams may be reduced and desirable monoalkylate product for gasoline blending may be produced via transalkylation in a fixed-bed reactor. For example, the benzene in light reformate may be transalkylated with the polyalkylate contained in heavy reformate.
Transalkylation refers generally to a type of chemical reaction that results in catalytic transfer of an alkyl group from a polyalkylate molecule, such as an aromatic hydrocarbon containing at least two alkyl groups, to a benzene molecule, to form monoalkylate product, an aromatic hydrocarbon containing only one alkyl group. Transalkylation may be used not only to reduce the content of benzene in gasoline feedstocks, but also to increase its octane rating while decreasing the content of polyalkylate, thus increasing the overall value of the fuel. A typical benzene transalkylation reaction is shown below.

As disclosed in U.S. Pat. No. 5,446,223, transalkylation reactions may utilize non-polluting, non-corrosive, regenerable materials, such as zeolitic molecular sieve catalysts. U.S. Pat. Nos. 4,371,714 and 4,469,908 disclose straight pass alkylation and transalkylation of aromatic compounds using zeolitic molecular sieve catalysts in fixed beds.
One problem with using a zeolitic catalyst in alkylation reaction is rapid deactivation of the zeolitic catalyst due to coking and poisoning, resulting in frequent unit shut downs or other process interruptions, such as for thermal regeneration of the catalyst.
The catalyst deactivation rate due to coking or poisoning may be reduced by maintaining the zeolitic catalyst in at least a partial liquid phase, such as a hydrocarbon slurry. U.S. Pat. Nos. 5,080,871 and 5,118,872 disclose a moving bed reactor for alkylation and transalkylation of aromatic compounds, in which a slurry is produced by adding solid catalyst to the aromatic feed stream and is circulated through the reactor.
One advantage of a moving bed catalyst slurry reactor, as taught by U.S. Pat. Nos. 5,080,871 and 5,118,872, is that the catalyst may be continuously replaced and regenerated during operation, thus reducing the need for unit shut downs. The ability to remove deactivated catalyst on-line may eliminate the need to remove catalyst poisons from the feeds or regenerate the catalyst as for a fixed bed reactor, thus reducing the cost of the benzene removal unit.
To date, benzene removal from reformate by transalkylation has not been found economical, generally because of the costly equipment required to remove poisons from the liquid and gas streams and the duplication of reactors for catalyst regeneration. Therefore, there is still a significant need in the art for economical methods to reduce the levels of benzene and C10 and heavier polyalkylate in refinery streams for smaller refining operations.